1. Field of the Invention
This invention generally relates to methods for fabricating microvalves. More specifically, the invention relates to procedures for forming a contoured valve stop in a microvalve.
2. Background Art
Microvalves may be used to control the flow of a gas in high temperature, high corrosive environments. In one such microvalve, a thin diaphragm, sandwiched between adjacent SiC or Si layers or wafers, is used to control the flow of gas through the valve. In use, the diaphragm abuts against a seat around an inlet of the valve to close that inlet; and when the pressure of gas in this inlet rises above a given value, the diaphragm flexes upwards, allowing gas flow through the inlet and through the valve.
A cavity or recess is provided inside the valve to allow this upward flexing movement of the diaphragm; and as the diaphragm flexes upward, the diaphragm comes into contact with surfaces of this cavity. This contact may cause significant localized forces or stresses on the diaphragm, or specific portions of the diaphragm.
Shaping this cavity or recess in any particular way is difficult because of the very small dimensions involved. For instance, this cavity may have a width or diameter of about one millimeter and a depth of about 0.025 millimeters. In the past, reactive ion etch (RIE) has been used to create very small, three-dimensional features in SiC and/or Si wafers. RIE etches can provide excellent trenches with steep smooth side walls, but RIE does not lend itself to creating contoured shapes. Since RIE uses a metal mask, it precludes using a gray scale mask. In addition, RIE is time consuming (etch rate of 0.001 mm/min). Inductively coupled plasma (ICP) etch techniques can also make clean well-defined trenches, but these techniques also are not conducive to making sloped and contoured surfaces. Lastly, the microvalve features are much too small for mechanical machining.
An object of this invention is to provide an improved microvalve.
Another object of the present invention is to extend the life of a microelectromechanical structure (MEMS) microvalve diaphragm.
A further object of the invention is to provide a dome-shaped contoured valve stop, in a microvalve, to reduce stress concentrations at the edge of a thin diaphragm of the valve.
Another object of the invention is to form a dome-shaped contoured diaphragm stop in a microvalve, by combining a laser tool definition with CAD artwork which defines a laser path such that the resulting geometry has no sharp edges that would cause the diaphragm to tear or rupture.
These and other objectives are attained with a microvalve and a method of forming a diaphragm stop for a microvalve. The microvalve includes a first layer and a diaphragm member to control the flow of fluid through the microvalve. The method comprises the step of forming a contoured shaped recess extending inward from a surface of the layer by using a laser to remove material in a series of areas, at successively greater depths extending inward from said surface. Preferably, the recess has a dome shape, and may be formed by a direct-write laser operated via a computer aided drawing program running on a computer.
For example, CAD artwork files, comprising a set of concentric polygons approximating circles, may be generated to create the dome structure. The laser ablation depth can be controlled by modifying the offset step distance of the polygons and equating certain line widths to an equivalent laser tool definition. Preferably, the laser tool definition is combined with the CAD artwork, which defines a laser path such that the resulting geometry has no sharp edges that could cause the diaphragm of the valve to tear or rupture.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description, given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.